This invention relates to a process to remove surface coatings from a substrate and an apparatus to carry out the process.
As far as in the prior art CO2-lasers are used to remove surface coatings from contaminated substrates, continuous-wave lasers are used to achieve the removal of organic and metal-organic contaminants by burning or evaporation. Herein, very high surface temperatures arise which are tolerated at most by metallic surfaces without additional damage of the surface to be cleaned. Continuous-wave laser treatment of nonmetallic substrates, especially plastic or biological materials, results in a decarbonization which destroys the substrate. In the case of ceramics and similar substrates a fragmentation from the surface occurs because their poor heat conduction results in high local thermal stress.
Therefore the purification of surfaces with a continuous wave laser has been restricted to only a few special fields.
The object of the invention is to provide a process and an apparatus to removexe2x80x94in the frames of a surface cleaningxe2x80x94color or oxide coatings or equivalents even from non-metallic surfaces with continuous-wave lasers.
This object is solved with a process to remove a surface layer from a substrate by irradiating laser radiation, wherein the surface is scanned sequentially with the laser radiation being concentrated to a focus, before irradiating a predetermined surface element, a thin fluid film or a fluid droplet cover is applied which covers at least the surface element, and the laser beam influences the surface element only for a very short treatment interval, especially less than 10 ms, wherein by appropriate choice of the process parameters including power density, treatment interval length, thickness of the fluid film, surface adhesion of the fluid, absorption characteristics of the fluid for the effective laser wavelength, and absorption characteristics of the fluid vapor an explosive evaporation of the fluid film is effected which results in a flaking of the surface and an evaporation of loose residues of the surface layer.
Surprisingly the disadvantages of a continuous-wave CO2 laser for substrate purification can be avoided by a combination of three characteristics:
First, the laser radiation is no longer guided over a broad area of the coated surface but it is focussed at a very high degree (nearly diffraction-limited) to achieve very high power densities (higher than 50 kW/cm2, preferably 250 kW/cm2).
Furthermore, the surface to be cleaned is covered with a thin liquid or droplet film, respectively, preferably immediately prior to the application of the laser radiation. The liquid film is applied e.g. with a spray nozzle ejecting an aerosol consisting of a gas/liquid mixture and being directed to the surface to be worked. The pre-moistening of the surface to be worked prior to the laser cleaning has the following effects: Using an appropriate liquid, preferably water, the CO2-laser radiation is absorbed nearly completely by the liquid. Under very high power densities and a focus diameter normally not higher than 2/10 mm, the liquid locally evaporates in an explosion. As a consequence of the surface adhesion of the liquid to the coating and the shock waves from the explosive evaporation, the coating of the surface to be cleaned flakes off and is carried away with the vapor flowing away.
According to a further feature of the invention the highly focussed laser beam is advanced over the working surface with a high speed. Typical scanning speeds are clearly higher than 1 mm/s. The result is that a surface element to be cleaned is exposed to the intense laser radiation for only a very short time, especially less then 10 ms. This leads, on one hand, to the above mentioned explosive evaporation and has, on the other, the result that the change of the transmission state of the liquid film during the evaporation (water vapor is a scale more transparent for CO2 laser radiation than a water film) provides just an intermediate radiation exposure with drastically reduced power density. Therefore, on the one hand the actually treated surface element will not be overheated, and on the other the process parameters can be chosen in dependence on the characteristics of each surface to be cleaned such that the remaining contamination can just be thermally evaporated.
Besides the above described effects, the combination of the process elements has an additional important advantage: As a result of an appropriate choice of the fluid, the molecules of the fluid react with the radicals developped from the contaminants during the explosive evaporation thereof. In this way, toxic and aggressive burning products and radicals can be bound. The vapor cloud from the evaporated fluid (water), the evaporated surface contaminant and the radicals bound in the fluid preferably will be sucked out and absorbed in a well-known filter system.
By combining those features the release of reaction products being dangerous for the environment during the surface cleaning is prevented. Neither are volatile and toxic gases produced nor do chemical active residues remain on the surfacexe2x80x94contrary to prior art cleaning techniques.
For the surface moistening, preferably by means of application of an aerosol, very thin films of a preferred thickness of 0.05 to 0.5 mm are formed so that only an extremely small amount of fluid is needed. In a preferred embodiment 1 liter liquid is sufficient for 10 m2 surface to be cleaned. In a further preferred embodiment of the invention the used liquid is recycled by means of a liquid separator in the filter system.
Besides water, other fluids can be used as long as they servexe2x80x94exploiting the inventive ideaxe2x80x94as a catcher for radicals of exhaust gas products of the contaminant removal evaporating during the removal of the surface coating, and are suitable to moisten a surfacexe2x80x94especially even hydrophobic surfaces.
By means of the proposed process surface coatings can be removed from inorganic surfaces, especially ceramics, and organic, especially polymeric and biological, surfaces.
Below the implementation of the inventive process in an apparatus is described in more detail in connection with a preferred embodiment.
The radiation of the CO2 laser is guided to a working handpiece by means of a prior art multi-mirror articulated arm or prior art optical waveguides. The handpiece contains the collimating and focussing optics for the CO2 laser radiation on the one hand and a scanning mechanism on the other which mechanism allows to guide the focussed laser beam over the entire surface to be treated. The scanning mechanism is arranged such that no turn-back points exist where the scanning speed becomes zero, i.e. its arrangement is circular, ellipsoidal or in the form of Lissajous figures. The laser radiation is guided to the surface under an angle differing from 90xc2x0 to prevent flowing-back of the material being explosively evaporated from the surface against the focussing optics and to avoid damage of the optics. The spraying nozzle for the aerosol is arranged essentially paraxial, but under a small angle against the direction of the axis of incidence of the laser beam such that primarily only a limited area is moistened which area essentially matches with the surface element to be actually influenced by the scanning laser beam.
Both main parts, the laser optic including the scanning means and the aerosol nozzle, are placed in an exhaust cage so that they can be guided by moving means (typically two or three steerable rolls or preferably three moving ball-bearings) to work at a small distance to the working surface. By virtue of the exhaust power in the exhaust nozzle a strong lateral flow arises in the narrow gap between exhaust cage and surface. This contributes to the prevention of contamination of the working optics by the evaporated removed products. Furthermore, the exhaust cage is arranged such that it allows free sight at the working surface through suitable windows or by virtue of its production from a transparent material. The window material is selected to absorb the used laser radiation such thatxe2x80x94if the handpiece is used in accordance with the rulesxe2x80x94the whole laser apparatus is safe in itself and meets the industrial safety standards.
The whole arrangement is mounted on an ergonomically designed handle resulting in an ergonomically simple use, Additionally the operation elements to trigger the laser radiation, to adjust the exhaust capacity and to adjust the aerosol mix are arranged at this handle.
Incidentally, preferred developments of the invention are specified in the dependent claims or described below in the context of a description of a preferred embodiment of the invention as illustrated in the figures.